1. Technical Field
This invention relates in general to balancing devices that move at high rotational speeds, and in particular to an improved apparatus and method for precision balancing of spindles in high speed computer hard disk drives.
2. Description of Related Art
Generally, a digital data access and storage system consists of one or more storage devices that store data on storage media such as magnetic or optical data storage disks. In magnetic disk storage systems, a storage device is called a hard disk drive (HDD), which includes one or more hard disks and an HDD controller to manage local operations concerning the disks. Hard disks are rigid platters, typically made of aluminum alloy or a mixture of glass and ceramic, covered with a magnetic coating. Typically, two or three platters are stacked vertically on a common spindle that is turned by a disk drive motor at several thousand revolutions per minute (rpm).
The only other moving part within a typical HDD is the head assembly. Within most drives, one read/write head is associated with each side of each platter and flies just above or below the platter""s surface. Each read/write head is connected to a semi-rigid arm apparatus which supports the entire head flying unit. More than one of such arms may be utilized together to form a single armature unit.
Each read/write head scans the hard disk platter surface during a xe2x80x9creadxe2x80x9d or xe2x80x9cwritexe2x80x9d operation. The head/arm assembly is moved utilizing an actuator which is often a voice coil motor (VCM). The stator of a VCM is mounted to a base plate or casting on which is also mounted the spindle supporting the disks. The base casting is in turn mounted to a frame via a compliant suspension. When current is fed to the motor, the VCM develops force or torque which is substantially proportional to the applied current. The arm acceleration is therefore substantially proportional to the magnitude of the current. As the read/write head nears the desired track, a reverse polarity signal is applied to the actuator, causing the signal to act as a brake, and ideally causing the read/write head to stop directly over the desired track.
One major determinant in HDD performance is the rotational speed of the spindle. Increased rotational speed reduces the time required to access a given piece of data on the disks. Unfortunately, increased rotational speed also magnifies any residual imbalance that is present in the disk pack. The force generated by such imbalances is proportional to the square of the rotational speed. Thus, doubling the rotational speed of the spindle increases the imbalance force by four times.
Imbalance forces in HDD spindles have at least two deleterious effects. The first effect is the resulting vibration in either the HDD having the imbalance, or in an adjacent HDD if the imbalanced HDD is operating in an array of drives. Vibration reduces the track-following capability of drives and, thus, their performance. The second effect of imbalance forces is the increase in acoustics that results from an imbalance force interacting with the structure of the system using the imbalanced HDD.
In the prior art, several solutions have been devised to balance HDD""s. Most of the solutions utilize some type of discrete elements that are selectable by mass. The discrete elements are carefully positioned about the top and/or bottom of the spindle as needed to correct the imbalance in top or bottom plane(s), respectively. Examples of the discrete elements used by these solutions include: balls, wire or flat sheet metal clips that are inserted into grooves; extra screws that are selectively fastened to an array of holes in the spindle; and lumps of heavy adhesive applied to the spindle at appropriate locations. One general problem with each of these methods is the inherent, limited resolution of discrete elements. The balance-correcting resolution of these methods is limited by the increments available between the elements, and by the range of masses available to the technician correcting the imbalance. Other problems with these solutions include the following: potential drive contamination concerns due to the additional holes or apertures in the spindle; snapping balls into grooves requires very precise control of the ball size and groove geometry; and the messiness of adhesives during the manufacturing process along with rework difficulties.
At least one prior art solution avoids the use of discrete elements altogether. This method detects the imbalance of the spinning disks and taps on the disk drive enclosure to carefully shift the disks with respect to the center of rotation of the spindle. Although tapping on the drive enclosure has theoretically infinite resolution, it is limited to correcting imbalances in a single plane. Thus, an improved solution for correcting imbalances in HDD spindles is needed.
A balancing mechanism for the spindle of a computer hard disk drive uses two, substantially flat wire clips at each end of the disk pack. The clips are designed so that one clip in each pair nests concentrically inside the other clip in the pair. Each clip is provided with a substantially identical imbalance in the installed position. If the imbalance vectors of each clip at one end of the disk pack are oriented 180 degrees apart, the resulting imbalance at that end of the pack is zero. Conversely, if the imbalance vectors are oriented at the same angle, the imbalance is maximized. Thus, the resolution of the imbalance provided by the clips is theoretically infinite between these limits, up to the precision of the clips. The orientation of the imbalance vector is controlled by the angle of the pair of clips relative to an index mark on the disk pack. The magnitude of the desired counterbalance is controlled by rotating the clips in each pair relative to each other. The cross-sectional shape of each pair of clips can be varied, and include the following examples: rectangular cross-sections having a generally flat profile, cross-sections with nesting grooves and ridges, and wire that is concave on one side and convex on the other side.